Transmission of data over long distances of optical fiber is limited by interference, such as from chromatic dispersion, which limits the usable bandwidth of the fiber. Chromatic dispersion is a result of the basic method by which fiber optic systems work. In particular, fiber optic transceivers work by transmitting “1” and “0” pulses, using two discrete levels of laser current to generate the two different pulses. But, the optical frequency of a semiconductor laser depends on the laser's current and the time derivative of the current. Ordinarily, this would not cause a problem, except that different optical frequencies travel at different velocities in an optical fiber. The result is that neighboring “1” and “0” pulses spread into one another. Over long distances of fiber, the effect from chromatic dispersion can become severe and the original data can no longer be recovered.
Chromatic dispersion is particularly an issue for 1550 nm laser light. This wavelength is used for long-distance transmission because it can be amplified by erbium-doped fiber or waveguide amplifiers, and because optical fiber has low attenuation at this wavelength. In contrast, laser light at 1310 nm, typically used for short-distance transmission, generally has very low chromatic dispersion in standard optical fiber.
Traditional prior-art solutions to chromatic dispersion generally fall into two classes: (i) limiting the optical frequency excursions (i.e., sidebands) of the transmitter, commonly known as “chirp”; and (ii) using special fiber- or optical-compensation elements so that different optical frequencies have the same transmit time from transmitter to receiver.
Examples of the specific technologies used to limit the chirp of a transmitter include: (1) special low-chirp or negative-chirp lasers that are designed to work at a fixed temperature maintained by a thermoelectric cooler; (2) externally-modulated lasers (EMLs); and (3) external modulation by lithium niobate or similar electro-optical modulators. However, these technologies generally add significant cost to a transceiver, as well as increased power consumption. Furthermore, it is theoretically impossible to completely remove chirp from a transmitter, since the modulation of an optical signal necessarily creates sidebands.
Examples of the specific technologies used as fiber- or optical-compensation elements include: (1) special low-dispersion fiber; (2) chirped fiber Bragg gratings; and (3) dispersion-compensating fiber. Like the technologies that limit chirp, these particular technologies are costly. They are also inconvenient for the customer to implement, and there is typically some residual dispersion penalty if the setup is not done perfectly.
Other types of dispersion that may occur in a fiber optic communication channel include polarization-mode dispersion and multimode-fiber dispersion.
One technique to reduce interference from dispersion is to utilize a linear equalizer, which is typically a transverse filter with multiple taps having various tap weights. The tap weights can be preset in the factory, or can be manually adjusted during installation. One potential problem with preset or manually-adjusted tap weights is that it may be difficult to optimize these parameters. Furthermore, manual adjustments cannot be used to compensate for variations in devices over time (e.g., with temperature), or to track polarization dispersion. These issues can be addressed, however, by using adaptive updating of the tap weights. An example of a well-known algorithm used to adaptively update tap weights is the least-mean-square algorithm. However, at the high-data rates of long-haul systems, the complex circuitry necessary to implement least-mean-square processing is complex, expensive, and, moreover, can consume an extensive amount of power.
Given the increasing speed of today's high-speed optical data communications system, with data rates of 5-10 Gb/s or even higher, and the increasing use of multiple channels on a single fiber optic, it is increasingly necessary to have an effective method to reduce interference in the optical signals. A technique that can reduce interference with minimal additional power requirements, and minimal additional manufacturing costs, would be highly desirable.